1. Field of the Invention
The present invention relates to the fabrication of plate fin heat exchange coils, and in particular to the ultrasonic soldering of return bends to straight heat exchange tubes to form one or more circuits in a plate fin coil.
2. Description of the Prior Art
A type of heat exchange apparatus widely used in heating and cooling air conditioning systems comprises a plurality of thin-walled straight tubes extending perpendicularly through a large number of parallel thin fins. The tubes are mounted in place by means of rigid end plates parallel with the fins and forming the opposite end walls of the plate fin coil. The tubes are interconnected to form one or more heat exchange fluid circuits. Pairs of tubes are connected at one end by "hair pins" which are generally integral with the straight tubes they connect. Pairs of tubes at the opposite end of the coil are connected by return bends, these being U-shaped tubes compatible with the straight tubes which they connect. In operation, heat exchange fluid such as a refrigerant, flows into an open end of a tube forming the beginning of a heat exchange tube circuit, proceeds through the circuit while heat is transferred between the fluid and the surroundings, and leaves the coil through another open tube forming the other end of the circuit.
The heat exchange tubes used in most plate fin coils are fabricated from aluminum because of the heat transfer and metallurgical characteristics of this material, and because of its low cost relative to other known heat conductors such as copper. Likewise, it has become conventional to fabricate the U-shaped return bends from aluminum tubing both because of the foregoing factors and because of the compatibility of aluminum return bends with aluminum straight tubes.
It is important that the return bends be connected to the respective straight tubes in a strong, fluid-tight manner, to prevent refrigerant or other heat exchange fluid from leaving the heat exchange circuit through any juncture of a return bend and the tubes to which it is connected. Although it is known to attach aluminum return bends to aluminum straight tubes by means of various brazing and conventional soldering techniques, those techniques have in general proven unacceptable. Current methods for connecting aluminum return bends to aluminum tubes involve ultrasonic soldering processes. These processes are particularly advantageous for soldering aluminum materials together, because a major impediment to such operations is the presence of the aluminum oxide coating which forms on raw aluminum surfaces exposed to oxygen as exists in the ambient air. This oxide coating severely hampers the establishment of a good solder joint, and the ultrasonic soldering process has been found to scrub this oxide coating from the aluminum surfaces being bonded, to enable a good solder bond to be effected. U.S. Pat. No. 3,752,381 describes an ultrasonic soldering apparatus useful for performing the bonding operation described herein, and commonly-assigned U.S. Pat. No. 3,760,481 describes an improved process for joining aluminum return bends to heat exchange tubes in plate fin coils. A common process for performing the ultrasonic soldering of aluminum return bends to aluminum heat exchange tubes in plate fin coils comprises the steps of prefitting the return bends into the bell-shaped open ends of the heat exchange tubes being connected, preheating the return bends and the heat exchange tubes in the region to be joined, immersing the prefitted return bends and the ends of the tubes into which they have been inserted into a molten solder bath to equalize the temperature across the area being soldered, applying ultrasonic energy to the molten solder, and withdrawing the plate fin coil from the solder bath to enable the solder to harden and form the desired bond.
In performing the preceding operation, it is extremely important that the air pressure within the tubes and return bends be relieved while the assembly is immersed in the molten solder, for otherwise the air pressure within the tubes and return bends would prevent the molten solder from flowing into the gap between the portion of the return bends inserted into the heat exchange tubes being joined, and the interior surfaces of those tubes opposite the inserted portions of the return bends. One procedure for venting the interior of a plate fin coil circuit to the atmosphere during the ultrasonic soldering process involves providing copper venting tubes soldered to the open ends of the respective circuits in the plate fin coil, these open ends being disposed at the same end of the coil as the return bends. A short extension tube or "stub out" is connected to the open ends of the aluminum heat exchange tubes, and a venting tube or "snorkel" is bonded to the "stub out" with a fluid tight bond and configured such that its open end is exposed to the ambient air when the "stub out" is immersed in the molten solder bath. After the completion of the ultrasonic soldering process, the snorkel tube is cut from the "stub out", and is discarded.
Although the foregoing venting apparatus and process effectively relieves the pressure in the respective circuits during the ultrasonic soldering process, it does suffer serious shortcomings. These snorkel tubes are not reuseable, and copper tubes are expensive items to discard. When the used copper snorkel tube is cut after the ultrasonic soldering process, copper chips occasionally are thrown onto the aluminum coil, and the presence of copper chips on the aluminum surface can result in electrolytic action between the two metals which may ultimately result in a leak in the aluminum tubes. Moreover, the time taken to bond the copper snorkel tube to the open aluminum tubes, and to subsequently cut those snorkel tubes after the soldering process, is very uneconomical, particularly in light of the high volume production of plate fin coils.